Conventionally, a variable focal lens (generally, which is sometimes called as a varifocal lens, and the same will be applied hereinafter) is known as a simplified zoom lens, and is used in a monitoring camera for example. While a normal zoom lens automatically adjusts its focus in accordance with zooming, a variable focal lens requires a readjustment of its focus upon zooming.
A focus adjustment of a variable focal lens is performed by manually rotating a focusing ring which is provided around a barrel of a camera to which the lens is mounted. However, the manual rotation of the focusing ring sometimes only results in an inadequate focus adjustment. For example, assume that a variable focal lens is mounted to a monitoring camera. In an installation, the monitoring camera is often set at a place remote from a monitor. In this case, one worker operates the camera while the other worker watches the monitor, so that the two workers cooperate for a focus adjustment. Thus, it is not easy to obtain an accurate focus in the installation. As another example, assume that a monitoring camera is installed during day time. In comparison between day time and night time, the depth of field is narrower during night time. So, even if an accurate focus is manually obtained during day time, the focus may be offset during night time.
Therefore, a variable focal lens preferably has an automatic focusing function. A conventional automatic focusing function applied to a variable focal lens is structured to detect a high frequency component of an image signal during a movement of a focus ring, and to stop the focusing ring at a position where the maximum value of the high frequency component is obtained. Such automatic focusing technology is disclosed in Japanese Patent Application Laid-Open No. 6-217180, for example.
FIG. 12 shows an conventional imaging apparatus having a variable focal lens which is provided with a automatic focusing function. In FIG. 12, an imaging apparatus 101 includes, as a structure for image pickup, a zoom ring 103, a focusing ring 105, an imaging element 107, a camera DSP 109, and a video circuit 111. The zoom ring 103 and the focusing ring 105 are provided with a plurality of lenses which form a subject image onto the imaging element 107, where the subject image is converted into electrical signals. The output signals from the imaging element 107 are supplied to the video circuit 111 after a processing at the camera DSP 109.
The zoom ring 103 and the focusing ring 105 can be manually operated separately. In operation, each ring is rotated around an optical axis, which causes a lens which is supported by the ring to move along the optical axis. A movement of the zoom ring 103 achieves a zooming, thereby an angle of view is adjusted. A movement of the focusing ring 105 achieves a focus adjustment.
For an automatic focusing function, the imaging apparatus 101 includes a stepper motor 113, a microcomputer 115, and an origin sensor 117.
The stepper motor 113 is provided to the focusing ring 105 so as to move the focusing ring 105 to a FAR side and a NEAR side. The stepper motor 113 is controlled by the microcomputer 115.
The microcomputer 115 has an up/down counter incorporated therein, so that a count value by the up/down counter is used to control the rotation of the stepper motor 113 to control the position of the focusing ring 105.
The origin sensor 117 detects the arrival of the focusing ring 105 at a predetermined position of an origin. Based on an output from the origin sensor 117, the microcomputer 115 detects an absolute position of the focusing ring 105. The origin sensor 117 is typically a photo interrupter type sensor, and detects the presence/absence of a shield which is provided to the focusing ring 105. The shield is arranged so that the connection of the origin sensor 117 is switched between both sides of the origin. This arrangement allows the origin sensor 117 to detect which side of the origin the focusing ring 105 is located on.
The microcomputer 115 controls the position of the focusing ring 105 based on a focus evaluation value which is supplied from the camera DSP 109. The focus evaluation value is a signal which represents the value of a high frequency component of image signals, and the camera DSP 109 produces a focus evaluation value from an output signal of the imaging element 107. The microcomputer 115 causes the focusing ring 105 to move to a focus alignment position (a position which achieves an accurate focus, or provides optimum focus, the same will be applied hereinafter) based on a change of the focus evaluation value due to the movement of the focusing ring 105. The focus alignment position is located at a position where the high frequency component of the image signals is the maximum.
FIG. 13A to FIG. 13C show auto-focus controls of a conventional variable focal lens. In each figure, a position of a focusing ring and its change are shown, and a NEAR side is on the left side and a FAR side is on the right side. FIG. 13A to FIG. 13C show examples having different start positions of the focusing ring 105 and different focus alignment positions (the positions after auto focus). In FIG. 13A, FIG. 13B, and FIG. 13C, the start position and the focus alignment position are located in the middle of a focusing ring movement range, near the FAR end, and near the NEAR end, respectively.
In the conventional auto-focus control, as shown in the FIG. 13A to FIG. 13C, ends provided by software are set between mechanical ends. The mechanical ends correspond to the ends of the focusing ring movement range, and are the positions where the rotation of a motor is mechanically limited. The software ends correspond to the ends of a focusing ring moving range which are set on the software of the microcomputer 115 to prevent the focusing ring 105 from reaching the mechanical ends.
In the auto-focus control, first, the microcomputer 115 determines whether the focusing ring 105 is located on the FAR side or on the NEAR side relative to the origin sensor 117, based on output of the origin sensor 17. Then, the microcomputer 115 causes the focusing ring 105 to move toward the origin sensor 117. In the examples of FIG. 13A and FIG. 13B, the focusing ring 105 is moved toward the NEAR side, and in the example of FIG. 13C, the focusing ring 105 is moved toward the FAR side. In order to eliminate backlash, an origin search operation is surely performed in the direction to the FAR side, and at the moment when the operation passes the origin, the up/down counter is reset.
Next, the microcomputer 115 causes the focusing ring 105 to reciprocate between the software ends at a high speed while detecting a focus alignment position, so that the microcomputer 115 approximately detects the focus alignment position. At this time, in order to eliminate backlash, the detection is surely performed in the direction to the FAR side.
Furthermore, the microcomputer 115 causes the focusing ring 105 to reciprocate at a low speed this time across the focus alignment position which was detected by the previous movement at a high speed, so that the microcomputer 115 accurately detect the focus alignment position. Finally, the microcomputer 115 causes the focusing ring 105 to stop at the focus alignment position. At this point also, in order to eliminate backlash, the microcomputer 115 surely causes the focusing ring 105 to move in the direction to the FAR side before the stopping.
As described above, the conventional variable focal lens uses the origin sensor 117 to detect an origin so as to control an absolute position of the focusing ring 105.
In addition, in the conventional variable focal lens as described above, the software ends are set between the mechanical ends of the focusing ring 105, and the focusing ring 105 is controlled not to reach the mechanical ends. This configuration is provided to prevent a step out of the stepper motor 113 at the mechanical ends. The step out means that a control position of the stepper motor and an actual position of the stepper motor are offset from each other. If the focusing ring 105 reaches a mechanical end, any supply of a driving signal does not cause the stepper motor 113 to rotate, resulting in a step out. Once a step out is caused, the position of the stepper motor 113 and the count value of the up/down counter do not correspond to each other, and the absolute position cannot be found. Thus, in order to prevent a step out, the software ends are set between the mechanical ends, which in turn provides margins at both ends.
However, in the conventional variable focal lens, the margins should be provided at both ends of a focusing ring movement range as described above, and disadvantageously a substantial zoom factor has to be reduced as compared to an optical design zoom factor.
Moreover, in the conventional variable focal lens, an origin sensor should be provided, which increases a cost and makes a barrel structure complexed.
The present invention was made in view of the above described background, and one object of the present invention is to provide a variable focal lens which does not require a reduction of a zoom factor, and does not need an origin sensor.